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ESEEinfhrung in Software Engineering
2. Requirements Collection
Prof. O. Nierstrasz
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Oscar Nierstrasz
ESE Requirements Collection
ESE 2.2
http://en.wikipedia.org/wiki/Fair_use
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ESE Requirements Collection
ESE 2.3
Roadmap
> The Requirements Engineering Process> Use Cases
> Functional and non-functional requirements
> Evolutionary and throw-away prototyping
> Requirements checking and reviews
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ESE Requirements Collection
ESE 2.4
Sources
> Software Engineering, I. Sommerville, 7th Edn., 2004.> Software Engineering A Practitioners Approach, R. Pressman,
Mc-Graw Hill, 5th Edn., 2001.
> Objects, Components and Frameworks with UML, D. D'Souza, A.Wills, Addison-Wesley, 1999
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ESE Requirements Collection
ESE 2.5
Roadmap
> The Requirements Engineering Process> Use Cases
> Functional and non-functional requirements
> Evolutionary and throw-away prototyping
> Requirements checking and reviews
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ESE Requirements Collection
ESE 2.6
Kommission:
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 88:00 - 9:00
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Bemerkungen: Unterschrift:
Zeitschema
Jan 2002 Feb 2002
Bitte ankreuzen wo Sie keinenfalls mitmachen knnen, und senden Sie das ausgefllte Formular bis ___________ ans Dekanat zurck.
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ESE Requirements Collection
ESE 2.7
Electronic Time Schedule
So,basically we need a form for the time schedule that can be distributed by eMail, aplace (html) where I can deposit these forms after they have been filled out, and an
algorithm that calculates a few possible meeting times, possibly setting priorities tocertain persons of each committee (since there will always be some time scheduleoverlaps). It would also be great if there were a way of checking whether everybody ofthe relevant committee has really sent their time schedule back and at the same timelisting all the ones who have failed to do so. An automatic invitation letter for thecommittee meeting to all the persons involved, generated through this programme, wouldbe even a further asset.
How can we transform this description into a requirements specification?
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ESE Requirements Collection
ESE 2.8
The Requirements Engineering Process
Feasibilitystudy
Requirementselicitation and
analysisRequirementsspecification
Requirementsvalidation
Feasibilityreport
Systemmodels
User and system
requirements
Requirements
document
Ian Sommerville 2000
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ESE 2.9
Requirements Engineering Activities
Feasibi l i ty studyDetermine if the user needscan be satisfiedwith the available technologyand budget.
Requirements
analysis
Find out what system stakeholders require
from the system.Requirements
def in i t ion
Define the requirementsin a formunderstandable to the customer.
Requirements
specif icat ion
Define the requirements in detail. (Written asa contract between client and contractor.)
Requirements are for users;
specifications are for analysts
and developers.
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ESE Requirements Collection
ESE 2.10
Requirements Analysis
Sometimes called requirements elicitationor requirementsdiscovery
Technical staff work with customers to determine
> the application domain,> the servicesthat the system should provide and
> the systems operational constraints.
Involves various stakeholders:> e.g., end-users, managers, engineers involved in
maintenance, domain experts, trade unions, etc.
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ESE Requirements Collection
ESE 2.11http://en.wikipedia.org/wiki/Fair_use
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ESE Requirements Collection
ESE 2.12
Problems of Requirements Analysis
Various problems typically arise:
Stakeholders dont knowwhat they really want
Stakeholders express requirements in their own terms
Different stakeholders may have conflicting requirements
Organisational and political factorsmay influence the systemrequirements
The requirements changeduring the analysis process.
New stakeholdersmay emerge.
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ESE Requirements Collection
ESE 2.13
Impedance Mismatches
As Management
requested it
As the Project
Leader defined it
As Systems
designed it
As Programming
developed it
As
Operations
installed it
What the
User wanted
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ESE 2.14
Requirements evolution
> Requirements always evolveas a better understanding
of user needs is developed and as the organisationsobjectives change
> It is essential toplan for changein the requirements as
the system is being developed and used
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ESE Requirements Collection
ESE 2.15
The Requirements Analysis Process
Requirementsvalidation
Domain
understanding Prioritization
Requirements
collection
Conflict
resolution
Classification
Requirementsdefinition andspecification
Processentry
Ian Sommerville 2000
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Oscar Nierstrasz
ESE Requirements Collection
ESE 2.16
Roadmap
> The Requirements Engineering Process
> Use Cases
> Functional and non-functional requirements
> Evolutionary and throw-away prototyping
> Requirements checking and reviews
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ESE Requirements Collection
ESE 2.17
Use Cases and Scenarios
A use caseis the specificationof a sequence of actions, includingvariants, that a system (or other entity) can perform, interacting withactorsof the system.
e.g., buy a DVD through the internet
A scenariois aparticular trace of action occurrences, starting from aknown initial state.
e.g., connect to myDVD.com, go to the search page
...
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ESE Requirements Collection
ESE 2.18
Use Cases and Viewpoints ...
Stakeholdersrepresent different problem viewpoints.
Interview as many differentkinds of stakeholders aspossible/necessary
Translate requirements into use casesor stories about thedesired system involving a fixed set of actors (users and systemobjects)
For each use case, capture both typical and exceptionalusagescenarios
Userstend to think about systems in terms of features.
You must get them to tell you storiesinvolving those features.
Use cases and scenarios can tell you if the requirements arecomplete and consistent!
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ESE Requirements Collection
ESE 2.19
Unified Modeling Language
Class Diagramsvisualize logical structureof system
in terms of classes, objects and relationships
Use Case Diagramsshow external actors and use casestheyparticipate in
Sequence Diagramsvisualize temporal message orderingof aconcrete scenarioof a use case
Collaborat ion
(Communicat ion)
Diagrams
visualize relationshipsof objects exchangingmessages in a concrete scenario
State Diagramsspecify the abstract statesof an object andthe transitionsbetween the states
UML is the industry standard for documenting OO models
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ESE Requirements Collection
ESE 2.20
Use Case Diagrams
More on
this later
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ESE Requirements Collection
ESE 2.21
Sequence Diagrams
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ESE Requirements Collection
ESE 2.22
Writing Requirements Definitions
Requirements definitions usually consist of natural language,supplemented by (e.g., UML) diagrams and tables.
Three types of problems can arise:
Lack of clarity: It is hard to write documents that are bothprecise andeasy-to-read.
Requirements confusion: Functional and non-functional requirementstend to be intertwined.
Requirements amalgamation: Several different requirementsmay beexpressed together.
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ESE 2.23
Roadmap
> The Requirements Engineering Process
> Use Cases
> Functional and non-functional requirements
> Evolutionary and throw-away prototyping
> Requirements checking and reviews
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ESE Requirements Collection
ESE 2.24
Functional and Non-functionalRequirements
Functional requirementsdescribe system servicesorfunctionsCompute sales tax on a purchase
Update the database on the server ...
Non-functional requirementsare constraintson the systemor the development process
Non-functional requirements may be more critical than functionalrequirements.
If these are not met, the system is useless!
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ESE 2.25
Non-functional Requirements
Product
requirements:
specify that the delivered product must behavein a particular way
e.g. execution speed, reliability, etc.
Organisat ional
requirements:
are a consequence of organisational policiesand procedures
e.g. process standards used, implementationrequirements, etc.
External
requirements:
arise from factors which are externalto thesystem and its development process
e.g. interoperability requirements, legislativerequirements, etc.
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ESE 2.26
Types of Non-functional Requirements
Performancerequirements
Spacerequirements
Usability
requirements
Efficiencyrequirements
Reliabilityrequirements
Portabilityrequirements
Interoperabilityrequirements
Ethicalrequirements
Legislative
requirements
Implementation
requirements
Standards
requirements
Delivery
requirements
Safetyrequirements
Privacyrequirements
Productrequirements
Organizationalrequirements
Externalrequirements
Non-functionalrequirements
Ian Sommerville 2000
S C
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ESE 2.27
Examples of Non-functionalRequirements
Product
requirement
It shall be possible for all necessary communicationbetween the APSE and the user to be expressed inthe standard Ada character set.
Organisat ional
requirement
The system development processand deliverabledocuments shall conform to the process anddeliverables defined inXYZCo-SP-STAN-95.
External
requirement
The system shall provide facilities that allow any
user to check if personal data is maintained on thesystem.A procedure must be defined and supportedin the software that will allow users to inspect
personal dataand to correct any errors in that data.
ESE R i C ll i
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ESE 2.28
Requirements Verifiability
Requirements must be written so that they can beobjectively verified.
Imprecise:
The system should be easy to useby experienced controllersand should be organised in such a way that user errors areminimised.
Terms like easy to use and errors shall be minimised areuseless as specifications.
Verif iable: Experienced controllers should be able to use all the system
functions after a total of two hours training. After this training,the average number of errors made by experienced usersshould not exceed two per day.
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ESE Requirements Collection
ESE 2.30
Precise Requirements Measures (II)
Property Measure
Reliability
Mean time to failure
Probability of unavailabilityRate of failure occurrence
RobustnessTime to restart after failurePercentage of events causing failureProbability of data corruption on failure
PortabilityPercentage of target dependent statementsNumber of target systems
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ESE Requirements Collection
ESE 2.31
Roadmap
> The Requirements Engineering Process
> Use Cases
> Functional and non-functional requirements
> Evolutionary and throw-away prototyping
> Requirements checking and reviews
ESE Requirements Collection
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ESE Requirements Collection
ESE 2.32
Prototyping Objectives
The objective of evolutionary prototypingis to deliver aworking systemto end-users.
Development starts with the requirements that are bestunderstood.
The objective of throw-away prototypingis to validate orderive the system requirements.
Prototyping starts with that requirements that arepoorlyunderstood.
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ESE Requirements Collection
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Evolutionary Prototyping
> Must be used for systems where the specification cannotbe developed in advance.
e.g., AI systems and user interface systems
> Based on techniques which allow rapid system iterations.e.g., executable specification languages, VHL languages, 4GLs,
component toolkits
> Verification is impossibleas there is no specification.Validationmeans demonstrating the adequacy of the system.
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ESE Requirements Collection
ESE 2.34
Throw-away Prototyping
> Used to reduce requirements riskThe prototype is developedfrom an initial specification, delivered
for experiment then discarded
> The throw-away prototype should notbe considered as afinal systemSome system characteristics may have been left out
(e.g., platform requirements may be ignored)
There is no specification for long-term maintenance
The system will be poorly structured and difficult to maintain
ESE Requirements Collection
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Oscar Nierstrasz
ESE Requirements Collection
ESE 2.35
Roadmap
> The Requirements Engineering Process
> Use Cases
> Functional and non-functional requirements
> Evolutionary and throw-away prototyping
> Requirements checking and reviews
ESE Requirements Collection
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ESE Requirements Collection
ESE 2.36
Requirements Checking
ValidityDoes the system provide the functionswhich best supportthe customers needs?
Consistency Are there any requirements conflicts?
CompletenessAre all functionsrequired by the customerincluded?
Real ismCan the requirements be implementedgiven available budget and technology?
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ESE Requirements Collection
ESE 2.37
Requirements Reviews
> Regular reviewsshould be held while the requirementsdefinition is being formulated
> Both client and contractorstaff should be involved inreviews
> Reviews may be formal(with completed documents) orinformal.
Good communicationsbetween developers, customers andusers can resolve problems at an early stage
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ESE 2.39
Traceability
To protect against changes you should be able to trace back from everysystem component to the original requirementthat caused itspresence
C1 C2 Cmreq1 x
req2 x
x
x
reqn x x
A software processshould help you keepthis virtual table up-to-date
Simple techniquesmay be quitevaluable (namingconventions, ...)
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ESE 2.40
What you should know!
> What is the difference between requirements analysisand specification?
> Why is it hard to define and specify requirements?
> What are use cases and scenarios?> What is the difference between functional and non-
functional requirements?
> Whats wrong with a requirement that says a product
should be user-friendly?> Whats the difference between evolutionary and throw-
away prototyping?
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Oscar Nierstrasz ESE 2.41
Can you answer the followingquestions?
> Why isnt it enough to specify requirements as a set ofdesired features?
> Which is better for specifying requirements: naturallanguage or diagrams?
> How would you prototype a user interface for a web-based ordering system?
> Would it be an evolutionary or throw-away prototype?
> What would you expect to gain from the prototype?
> How would you check a requirement for adaptability?
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